In the speech, Younger said the UK had partly trusted Russian President Vladimir Putin when Russia pardoned Skripal in 2010 in return for its own agents.

Younger said he and his agents assumed that Moscow’s spy swap “had meaning” and would be honored, but he said they revised their opinion in light of the Skripal attack.

He said, according to the Financial Times: “Mr. Skripal came to the UK in an American-brokered exchange, having been pardoned by the president of Russia and, to the extent we assumed that had meaning, that is not an assumption that we will make again.”

Spy swaps

Spy swaps are understandings between the West and Russia that date back to the of the Cold War.

Jonathan Eyal, international director at think-tank the Royal United Services Institute (RUSI), told Business Insider in March that the safety of the spies is typically put at the forefront of the exchange.

“Spying agencies try to maintain a gentleman’s agreement that these people are beyond retribution,” he said.

The goal is typically to have them go smoothly so more spy swaps can be done in the future.

Professor Anthony Glees, the director of the Centre for Security and Intelligence Studies at the University of Buckingham, told Business Insider that the Russians take spy swaps “very seriously” because of the concern that “no one will ever do a swap with them again” if they break faith.

He said if Russia had really wanted to kill Skripal, the country could have executed him in prison.

So Russia would have needed to believe it had a good reason to attempt to assassinate Skripal on UK soil.

“The idea that they would do it for fun or anything less serious is to be discounted,” Eyal said.

A state of confrontation

Speaking on Monday, Younger said Russia was in a “perpetual state of confrontation” with the UK and warned the Kremlin not to underestimate the UK’s determination to fight attempts to interfere with its way of life.

“The conclusion [Russia has] arrived at is they should apply their capabilities across the whole spectrum to … our institutions and our partnerships,” Younger said.

“Our intention is for the Russian state to conclude that whatever benefits it thinks it is accruing from this activity, they are not worth the risk.”